1. Field of the Invention
The present invention relates to a counter balance valve, and more particularly to a counter balance valve mounted on a driving hydraulic circuit or the like of an oil hydraulic motor for a running unit of a construction machine.
2. Description of the Related Art
For example, a known driving hydraulic circuit of the oil hydraulic motor is shown in FIG. 11. In this driving hydraulic circuit, discharge passage la of an oil hydraulic pump 1 is connected to first main circuit 3 and second main circuit 4 through an operation valve 2. First main circuit 3 and second main circuit 4 are respectively connected to first port 6.sub.1 and second port 6.sub.2 of oil hydraulic motor 5. Counter balance valve 7 is disposed between first main circuit 3 and second main circuit 4.
In such a driving hydraulic circuit, when operation valve 2 is in neutral position N, counter balance valve 7 also falls in neutral position N, circuits of first and second main circuits 3, 4 which are closer to oil hydraulic motor 5 than check valves 8 of first and second main circuits 3, 4 are closed to prevent oil hydraulic motor 5 from being rotated by an external force.
Where operation valve 2 is set to first position (1) or second position (2), a pressure oil is supplied to either of first or second main circuits 3, 4, and counter balance valve 7 is switched to first position A or second position B by the pressure oil.
Counter balance valve 7 used for the driving hydraulic circuit is switched to first position A as the high pressure oil of first main circuit 3 acts on left pressure reception chamber 7a, and also switched to second position B as the high pressure oil of second main circuit 4 acts on right pressure reception chamber 7b. As a result, either of second or first main circuit 4, 3 is connected to tank 9 through counter balance valve 7.
On the other hand, when the high pressure oil is removed, counter balance valve 7 is pushed toward neutral position by springs 7c so to return to neutral position N while discharging the oil from left pressure reception chamber 7a or right pressure reception chamber 7b.
When oil hydraulic motor 5 is to be stopped, it is rotated by inertia to make pumping.
Therefore, when oil hydraulic motor 5 is stopped with operation valve 2 at neutral position N, counter balance valve 7 immediately comes to neutral position N, and either of first port 6.sub.1 and second port 6.sub.2 rapidly becomes a high pressure, resulting in having a large shock when oil hydraulic motor 5 is stopped.
In order to decrease a shock when oil hydraulic motor 5 is stopped, a speed of returning counter balance valve 7 from first position A or second position B to neutral position N may be decreased. In other words, the pressure oil in first port 6.sub.1 and second port 6.sub.2 may be discharged into tank 9 by closing counter balance valve 7, so that the shock involved in stopping oil hydraulic motor 5 can be decreased. For example, as to counter balance valve 7 described above, chokes 11, 11 are respectively mounted on circuit 10 which connects left pressure reception chamber 7a with first main circuit 3 and another circuit 10 connecting right pressure reception chamber 7b with second main circuit 4. Thus, the oil is discharged slowly from left pressure reception chamber 7a and right pressure reception chamber 7b by decreasing opening areas chokes 11, 11. As a result, counter balance valve 7 is made to slowly return from first position A or second position B to neutral position N, and the shock involved in stopping oil hydraulic motor 5 can be decreased.
However, the above configuration makes the counter balance valve 7 takes a longer time to return to neutral position N. Therefore, it takes a lot of time before oil hydraulic motor 5 stops completely. At this time, because relief valve for recirculating from first port 6.sub.1 to second port 6.sub.2 does not open, a flow rate becomes insufficient due to suction from tank toward lower pressure side, a cavitation may occur.
In order to solve such a conflicting problem of a shock at the high-pressure side and a cavitation at the low-pressure side, a counter balance valve is described in Japanese Patent Application Laid-Open No. 1-101708. FIG. 12 shows this counter balance valve.
The counter balance valve 7 maintains a spool in the neutral position by a spring 7c. The left pressure reception chamber 7a is communicated with a first pump port 12 and shifts the spool to the first position A when the pressure oil supplied. The right pressure reception chamber 7b is communicated with a second pump port 13 and shifts the spool to the second position B when the pressure oil is supplied.
A passage from the first pump port 12 to the left pressure reception chamber 7a and a passage from the second pump port 13 to the right pressure reception chamber 7b respectively have a first choking hole 14, a second choking hole 15 and a third choking hole 16.
In this counter balance valve 7, for example when the spool moves from the first position A to the neutral position N, at the beginning of a stroke to the intermediate position, the pressure oil in the left pressure reception chamber 7a flows smoothly to the first pump port 12 through the first choking hole 14 and the second choking hole 15 (a large communication area). When the spool has stroked to the intermediate position N, the pressure oil in the left pressure reception chamber 7a flows through the first choking hole 14 (a small communication area) only. When the spool further strokes, it is designed that the pressure oil in the left pressure reception chamber 7a flows through the first choking hole 14 and the third choking hole 16 (an intermediate communication area). The same is also applied when the spool is moved from the second position B to the neutral position N.
In this counter balance valve 7, the moving speed from the first position A or the second position B to the neutral position N is high at the beginning of a stroke, low at the middle of the stroke and intermediate at the end of the stroke. As a result, the counter balance valve 7 allows to return to the neutral position N in a short time while preventing cavitation from occurring.
Accordingly, the counter balance valve 7 mounted on the driving hydraulic circuit of the oil hydraulic motor 5 makes it possible to decelerate to stop the oil hydraulic motor 5 in a short time while preventing cavitation from occurring.
When a pressure in the left pressure reception chamber 7a drops and the spool is moved from the first position A to the neutral position N by the spring force, the counter balance valve 7 has the left pressure reception chamber 7a communicated with the first pump port 12 through the first choking hole 14 (a small communication area) at the middle of the stroke described above. During which the right pressure reception chamber 7b is kept communicated with the second pump port 13 through the first choking hole 14 and the third choking hole 16. In other words, the communication area between the right pressure reception chamber 7b and the second pump port 13 is identical with the intermediate communication area communicating between the left pressure reception chamber 7a and the first pump port 12 at the end of the stroke.
Thus, when the counter balance valve 7 is continuously used with the spool at the middle position of the stroke as described above, an effect of preventing vibrations while the spool moves from the first position A to the neutral position N is enhanced when the running unit of a construction machine is going downhill, for example.
However, when the spool is moved from the neutral position N toward the first position A, the pressure oil of the right pressure reception chamber 7b flows smoothly to the second pump port 13, hence a vibration preventing effect is lowered. As a result, an over-stroke of the spool may be caused when the spool moves toward the first position A.
For this reason, a drive torque of the oil hydraulic motor 5 is varied due to the road irregularities during running down hill and thus a pressure change of the return pressure oil of the oil hydraulic motor 5 becomes large, the spool is moved because the vibration prevention effect of the spool is small and a change in the opening area between the pump port 12, 13 and the motor port 17,18 becomes large. As a result, it may cause a change (hunching) in the rotating speed of the oil hydraulic motor 5 or a scratching-like movement, that is, an irregular speed change of the oil hydraulic motor 5.
The same thing is also caused when the spool is moved from the second position B to the neutral position N.